Select pass mode via python script
With the help of @B.Y.O.B. I realized that it's not possible to change "pass" setting, because LuxCore (and also Cycles) processes all selected passess (AOVs) while rendering, so there is no point in changing that variable with python (therefore it's not exposed to the API). 

Anyway, my final goal was to output only certain pass (in my case "Irradiance"), which I achieved with standard way for manipulating render output - Compositor Node Editor. I used File Output node as shown on my code example:

import bpy

bpy.context.area.type = 'NODE_EDITOR'

bpy.context.space_data.tree_type = 'CompositorNodeTree'
bpy.context.scene.use_nodes = True

C = bpy.context
node_tree = C.scene.node_tree

render_layers = node_tree.nodes.get(Render Layers)

# This command creates File Output node
node_tree.nodes.new(type=CompositorNodeOutputFile)
output_file_node = node_tree.nodes.get(File Output)
output_file_node.base_path = "save path"

# Link Render Layers to File Output node.
irradiance_output = render_layers.outputs.get(IRRADIANCE)
link = node_tree.links.new(irradiance_output, output_file_node.inputs[0])